The present invention relates generally to fuel cells and relates more particularly to a novel fuel cell system.
Fuel cells are electrochemical devices in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. Because of their comparatively high inherent efficiencies and comparatively low emissions, fuel cells are presently receiving considerable attention as a possible alternative to the combustion of nonrenewable fossil fuels in a variety of applications.
A typical fuel cell comprises a fuel electrode (i.e., anode) and an oxidant electrode (i.e., cathode), the two electrodes being separated by an electrolyte that is a good conductor of ions but a poor conductor of electrons. The electrodes are connected electrically to a load, such as an electronic circuit, by an external circuit conductor. Oxidation of the fuel at the anode produces electrons that flow through the external circuit to the cathode producing an electric current. The electrons react with an oxidant at the cathode. In theory, any substance capable of chemical oxidation that can be supplied continuously to the anode can serve as the fuel for the fuel cell, and any material that can be reduced at a sufficient rate at the cathode can serve as the oxidant for the fuel cell.
In one well-known type of fuel cell, sometimes referred to as a hydrogen fuel cell, gaseous hydrogen serves as the fuel, and gaseous oxygen serves as the oxidant. The electrodes in a hydrogen fuel cell are typically porous to permit the gas-electrolyte junction to be as great as possible. At the anode, incoming hydrogen gas ionizes to produce hydrogen ions and electrons. Since the electrolyte is a non-electronic conductor, the electrons flow away from the anode via the external circuit, producing an electric current. At the cathode, oxygen gas reacts with hydrogen ions migrating through the electrolyte and the incoming electrons from the external circuit to produce water as a byproduct. The overall reaction that takes place in the fuel cell is the sum of the anode and cathode reactions, with part of the free energy of reaction being released directly as electrical energy and with another part of the free energy being released as heat at the fuel cell. Although the electrolyte of a fuel cell may be a liquid electrolyte, more commonly the electrolyte of a fuel cell is a solid polymer electrolyte or proton exchange membrane (PEM). One of the more common types of PEMs is a perfluorosulfonic acid (PFSA) polymer, said PFSA polymer being formed by the copolymerization of tetrafluoroethylene and perfluorovinylether sulfonic acid. Often, a number of fuel cells are assembled together in order to meet desired voltage and current requirements. One common type of assembly, often referred to as a bipolar stack, comprises a plurality of stacked fuel cells that are electrically connected in series in a bipolar configuration.
Most fuel cells are run using a finite quantity of fuel, the fuel typically being withdrawn from a storage vessel as needed. For example, in the case of a hydrogen fuel cell, hydrogen gas is typically stored in and withdrawn from a hydrogen storage tank. As can be appreciated, if fuel is withdrawn from a storage vessel, and the fuel is not replenished thereafter in some manner, then eventually there will be no fuel left for the fuel cell to operate. A regenerative fuel cell system addresses this problem by including equipment that may be used to regenerate fuel for the fuel cell. For example, in the case of a hydrogen fuel cell system, the equipment for regenerating fuel may include an electrolyzer that is run to convert water into oxygen gas and hydrogen gas. The electrolyzer may be operated using solar, wind or geothermal energy so as not to deplete the electrical energy produced by operation of the fuel cell. In this manner, a regenerative fuel cell system may be used in a fashion similar to a rechargeable battery, with the electrolyzer being run to store energy and with the fuel cell being run to generate electrical current. A regenerative fuel cell system may include separate electrolyzer and fuel cell units or may include a bifunctional unit that may be alternately operated either as an electrolyzer or as a fuel cell. In those instances in which a bifunctional unit is used, the system is typically referred to as a unitized regenerative fuel cell system. Regenerative fuel cell systems may be either closed-loop, in which case the quantities of fuel, oxidant and products are limited, or open-loop, in which case the quantities are unlimited.
Additional background information relating to regenerative fuel cell systems may be found, for example, in the following patents and publications, all of which are incorporated herein by reference: U.S. Pat. No. 6,887,601 B2, inventors Moulthrop, Jr. et al., issued May 3, 2005; U.S. Pat. No. 6,838,205 B2, inventors Cisar et al., issued Jan. 4, 2005; U.S. Pat. No. 6,833,207 B2, inventors Joos et al., issued Dec. 21, 2004; U.S. Pat. No. 3,981,745, inventor Stedman, issued Sep. 21, 1976; Giner et al., “Fuel Cells As Rechargeable Batteries,” Proceedings NATO-ARW, Kiev 5/95 (Kluwer, Dordrecht, 1/96) pp. 215-232; Burke, “High Energy Density Regenerative Fuel Cell Systems for Terrestrial Applications,” IEEE AES Systems Magazine, 23-34 (1999); and Ioroi et al., “Thin film electrocatalyst layer for unitized regenerative polymer electrolyte fuel cells,” Journal of Power Sources, 112:583-7 (2002).
One problem that is commonly encountered with hydrogen fuel cells of the type having a proton exchange membrane is that water tends to accumulate on the membrane, particularly on the oxygen side of the membrane where water is produced. This is problematic because the accumulated water often impedes the delivery of gases to the membrane. To counter this problem, gas flows are often used that are in excess of what is required stoichiometrically so that the excess gas may be used to transport the accumulated water away from the membrane. However, as can be appreciated, where the quantity of oxygen is limited, such as in a closed-loop regenerative fuel cell system, the use of excess oxygen is problematic. To address this problem, the excess oxygen from a fuel cell or, more typically, a fuel cell stack, is typically recycled. However, because there is a pressure drop from the fuel cell stack oxygen inlet to the fuel cell stack oxygen outlet, the recycled oxygen is typically re-pressurized using a gas compressor before joining the oxygen inlet flow stream delivered to the fuel cell stack. The use of a compressor, however, creates its own problems. This is because the recycled oxygen has a high level of humidity; as a result, the compression of the humidified oxygen causes some water to condense in the compressor. However, such condensation of water in the compressor is undesirable as the condensed water adversely affects the performance of the compressor. Consequently, a gas dryer is typically used to remove some, but not all, of the water from the recycled oxygen before the recycled oxygen is sent to the compressor. (It is not desirable to remove all of the water from the recycled oxygen as a certain degree of humidification of the recycled oxygen is desirable to keep the membrane appropriately humidified.) Nevertheless, even using a dryer in the manner described above to remove some of the water from the recycled oxygen, the problem of water condensation in the compressor is not entirely eliminated.